Photovoltaic device

ABSTRACT

This invention is a layered thin film semiconductor device comprising a first transparent layer; a thin, second transparent layer having a conductivity less than the first transparent layer; an n-type layer; and a p-type layer comprising one or more IIB and VIA elements. This invention is also a method for making such semiconductor device. The thin film semiconductor devices of this invention are useful for making photovoltaic devices.

[0001] This application claims the benefit of Provisional PatentApplication No. 60/289,481, filed on May 8, 2001.

FIELD OF THE INVENTION

[0002] This invention relates to a new, thin film semiconductor device.More particularly, this invention relates to a new thin filmphotovoltaic semiconductor device having improved efficiency inconverting solar or other light energy into electrical energy. Thisinvention also relates to a method of manufacturing such semiconductorand photovoltaic device.

BACKGROUND OF THE INVENTION

[0003] Solar power is an important source of renewable electricalenergy. The continuous challenge in the field of solar energy is todevelop and manufacture photovoltaic devices having a high efficiencyfor converting sunlight into electrical energy. The more efficient thephotovoltaic device is at performing such a conversion, the greateramount of electricity can be generated for a given investment. While anumber of different types of photovoltaic devices have been developed, aparticularly suitable photovoltaic device is a thin film semiconductordevice having at least one layer, a p-layer, comprising one or more IIBelements and one or more VIA elements from the Periodic Table ofElements. One such photovoltaic device is referred to as a CdTe devicebecause the IIB and VIA elements are cadmium and tellurium,respectively. Typically, these photovoltaic devices also have an n-layeror window layer generally comprising cadmium sulfide. Such a device issometimes referred to as a CdS/CdTe device or cell. They also typicallyhave a transparent, electrically conductive first contact and a second,generally opaque, electrically conductive second contact. In a usualconfiguration, these devices have a first transparent conductive layerof conductive metal oxide which is a first electrical contact, ann-layer or window layer of the n-type comprising cadmium sulfidedeposited on the first transparent conductive layer, a p-layer depositedon the n-layer and a second, generally opaque electrical contactdeposited on the p-layer. The junction of the n-layer and the p-layer isa heterojunction, as is known in the art, and is responsible for thegeneration of electric potential and electric current when thesemiconductor device is exposed to light energy, such as sunlight. Lightenters the device from the side of the first transparent layer. Suchdevices have demonstrated superior efficiency and power generationcompared to other types of thin film photovoltaic devices. See forexample the article by D. Cunningham et al., “Large Area Apollo ModulePerformance and Reliability,” 28^(th) IEEE Photovoltaic SpecialistsConference, Anchorage, Ak., September 2000. However, while such CdTethin film, semiconductor photovoltaic devices are efficient and are alsoamenable to commercial manufacturing methods, the art needs suchphotovoltaic devices with improved efficiency. The present inventionprovides for such thin film, semiconductor photovoltaic devices havingimproved efficiency in converting sunlight into electric current.

SUMMARY OF THE INVENTION

[0004] This invention is a layered thin film semiconductor devicecomprising a first transparent layer; a thin, second transparent layerhaving a conductivity less than the first transparent layer; an n-typelayer; and a p-type layer comprising one or more IIB and VIA elements.This invention is also a method for making such semiconductor device.The thin film semiconductor devices of this invention are useful formaking photovoltaic devices.

BRIEF DESCRIPTION OF THE DRAWING

[0005]FIG. 1 shows the layered structure of one of the embodiments ofthis invention.

DETAILED DESCRIPTION OF THE INVENTION

[0006] This invention comprises a layered, thin film semiconductordevice suitable for generating electrical current upon exposure to lightenergy, particularly solar energy. In one embodiment, the inventedlayered, thin film device comprises a first transparent layer comprisinga conductive material, a thin second transparent layer having aresistivity more than the resistivity of the first transparentconductive layer, an n-type layer and a p-type layer; the p-type layercomprising one or more IIB and VIA elements of the Periodic Table ofElements. As used herein, the term n-type means a negatively dopedsemiconductor and the term p-type means a positively dopedsemiconductor. In a preferred embodiment, the layered, thin filmsemiconductor device is supported by or deposited on a suitablesubstrate material such as a glass, plastic or metal. Preferably thesubstrate material is glass, preferably a clear glass and preferably theglass is in the form of a flat sheet.

[0007] The substrate material is most preferably clear, flat glass, andcan have any shape but is generally square or more preferablyrectangular. The thickness of the glass can be any thickness thatprovides for the necessary support of the thin film semiconductor devicebut generally is about 0.2 to about 5 millimeters in thickness.

[0008] Preferably, the first transparent layer comprises one or moreconductive metal oxides such as tin oxide, zinc oxide, indium tin oxide,or a mixture of one or more of these oxides, or some other conductiveand transparent material. The first transparent layer is typically about0.4 to about 0.7 microns in thickness, more preferably about 0.45 toabout 0.5 microns in thickness. Suitable methods for forming the firstconductive layer include spray pyrolysis or chemical vapor deposition(CVD). The first transparent layer is suitably applied directly onto thesubstrate. A preferred first layer used in this invention is a layer oftin oxide applied to a glass substrate by CVD and further comprisingfluorine dopant to decrease the resistivity of the first transparentlayer to the desired resistivity. Preferably, the first transparentlayer has a resistivity of about 1×10⁻⁴ to about 5×10⁻⁴ ohm·cm. Flatglass substrates containing the first conductive layer comprising tinoxide doped with fluorine can be purchased from suitable glassmanufacturers and suppliers.

[0009] The second transparent layer of the thin film semiconductordevice of this invention is a thin layer having a resistivity more thanthe resistivity of the first transparent layer. By thin we mean,preferably, the layer has a thickness of up to about 0.075 microns, orup to about 0.065 microns, or up to about 0.05 microns, more preferablyabout 0.01 to about 0.05 microns, and still more preferably of about0.015 to about 0.035 microns. Preferably, the thin, second transparentlayer is at least about 0.01 microns in thickness. Preferably, thesecond transparent layer comprises one or more conductive metal oxidessuch as tin oxide, zinc oxide, indium tin oxide, zinc stannate, amixture of one or more of the above, or some other conductive andtransparent material. Preferably, the second transparent layer comprisesa metal oxide and is, preferably, the same metal oxide used in the firstconductive layer but is doped differently so that its more resistivethan the first transparent layer. Preferably, the second transparentlayer comprises a mixture of tin and zinc oxides. The molar ratio of tinoxide to zinc oxide in the mixture is suitably about 99.9:0.1 to about95:5, preferably about 99.5:0.5 to about 98:2, and more preferably about99.2:0.8 to about 98.8:1.2. The second transparent layer is preferablydeposited next to and in immediate contact with the first transparentlayer. The method used for depositing the second transparent layer isany suitable method that will provide the thin film of secondtransparent layer. For example, CVD, spray pyrolysis or physical vapordeposition can be used to form or deposit the second transparent layer.However, we discovered that the preferred method to form and deposit thesecond transparent layer is to use reactive direct current (DC)sputtering. In this process, a metal or a mixture of metals, for exampleelemental tin, elemental zinc, or mixtures thereof which may be inatomic ratios the same as the molar ratios mentioned above for tin oxideand zinc oxide, is sputtered onto the substrate. lndium can also be usedalone or in combination with either or both tin and zinc. In thepreferred method, such a mixture of metals is sputtered directly ontoand in direct contact with the first transparent layer. In the reactiveDC sputtering method, the metal or metals, for example, a mixture ofelemental tin and elemental zinc in an atomic ratio of about 99:1,respectively, are sputtered onto the substrate using, for example, anAirCo Thin Film Coater in an oxidizing atmosphere at a temperature ofabout 30° C. to about 90° C. and at a pressure of about 0.1 millitorr toabout 5 millitorr. The oxidizing atmosphere can be any atmosphere thatwill serve to convert the sputtered metal or metals to oxides during thesputtering process. For example, the oxidizing atmosphere can be amixture of oxygen gas with one or more inert gasses such as argon,helium or nitrogen such that the ratio of oxygen gas to inert gas isabout 99:1 to about 1:99. The oxidizing atmosphere can be oxygen gas.After the sputtering, the layer deposited is optionally heated in air oran oxidizing atmosphere, such as the oxidizing atmosphere describedabove, suitably at about 400° C. to about 500° C. for about 5 to about60 minutes, more preferably about 10 to about 30 minutes. Preferablyall, or substantially all, of the sputtered metal in the layer is in itsoxide form. By “substantially all” we mean, preferably, that at leastabout 98 percent, more preferably at least about 99 percent and mostpreferably at least about 99.9 percent of the metal in the depositedlayer is in the oxide form. We determined that the use of reactive DCsputtering as the method for depositing the second transparent layerprovides for a more efficient photovoltaic device of this inventioncompared, for example, to the same device where the second transparentlayer is applied by sputtering the metal followed by oxidation of thesputtered metal to the oxide by heating in air. Photovoltaic devices ofthis invention having a second transparent layer made by reactive DCsputtering a mixture of tin and zinc metal were also superior tophotovoltaic devices of this invention having a second transparent layermade by chemical vapor deposition of tin oxide or sputtered tin oxide.

[0010] The second transparent layer has a resistivity more than theresistivity of the first transparent layer. For example, the resistivityof the second transparent layer is up to about 1×10⁶ times moreresistive, preferably about 1×10⁴ to about 5×10⁵ times more resistivethan the first transparent layer. In absolute terms, the resistivity ofthe second transparent layer is suitably up to about 100 ohm·cm.,preferably about 1 to about 15 ohm·cm, and more preferably about 5 toabout 10 ohm·cm. The transparency of the second transparent layer issuitably at least about 90 percent, preferably at least about 95 percentand more preferably at least about 99 percent, as measured byspectrophotometric methods. The second transparent layer may betextured. Such texturing may be accomplished by, for example, modulatingthe deposition rate of the metal oxides, etching after deposition withan acid such as hydrofluoric or hydrochloric acid, or with a base suchas sodium hydroxide, or by plasma or reactive ion etching. Although thesecond transparent layer in the invented semiconductor device may becrystalline or polycrystalline, it may be at least partially amorphous,for example, at least about 50 weight percent amorphous. Preferably itis amorphous or substantially amorphous. By substantially amorphous wemean, preferably, that the layer is at least about 95 weight percentamorphous, more preferably at least about 98 weight percent amorphousand most preferably at least about 99 weight percent amorphous. Althoughthe second transparent layer has been described hereinabove as aseparate layer, it is to be understood that the second layer can becontinuous with the first layer in that instead of a discrete boundarybetween the first layer and second layer there is a continuous or gradedchange in composition so as to achieve the purposes of the firsttransparent layer and second transparent layer as described herein.Thus, for example, rather than depositing a first transparent layer thenswitching to deposit the second transparent layer, the composition ofthe feedstock used to deposit the first layer, for example, tin or tinoxide, can be switched during the deposition process for example, to amixture of tin and zinc or tin oxide and zinc oxide, so that althoughthere may not be a discrete separation of the two layers, the regionsdeposited will function in the same manner as discrete layers.

[0011] The n-layer in the thin film semiconductor device of thisinvention is any suitable material that provides for compatibility withand capable of forming a heterojunction with a suitable p-layer.Preferably, the n-layer forms a heterojunction with a p-layer,preferably a CdTe p-layer, that will generate electrical chargeseparation upon exposure to light energy, preferably solar light energy.Preferably, the n-layer, also known as the window layer, comprisescadmium sulfide (CdS), but it can also comprise zinc sulfide, cadmiumzinc sulfide, or one or more mixtures of any of the above. The CdS layercan be deposited by any suitable method. For example, it can bedeposited by contacting the substrate (having the second transparentlayer deposited on the first transparent layer) with an alkaline aqueousmedium comprising a source of cadmium ion, sulfide ion (or precursorthereof), a colloid stabilizer and, preferably, a complexing agent forcadmium ion. The cadmium source is usually a water-soluble salt orcomplex, for example, an inorganic salt such as cadmium chloride orsulfate, or organic salt, for example a carboxylate such as cadmiumacetate. The aqueous medium may contain about 0.3 to about 10,preferably about 0.5 to about 2.5, and more preferably about 1 to about5 grams per liter of cadmium. The sulfide is usually a hydrosulfide ionor preferably an inorganic or organic precursor thereof. The precursor,if used, is preferably water soluble, for example, to an extent of atleast about 1 or preferably at least about 10 grams per liter. Examplesof inorganic sulfide are hydrogen sulfide, metal sulfides includingalkali metal sulfide such as sodium sulfide, alkaline earth metalsulfide such as calcium sulfide, or a nonmetal or insoluble metalsulfide such as phosphorus pentasulfide or aluminum sulfide,respectively. Preferably the precursor is organic and is hydrolyzable,especially under alkaline conditions to give the sulfide ion. Examplesare thiocarbonyl compounds such as thio-ketones and aldehydes such asthioformaldehyde and thioacids and amides thereof especially thio acidsand amides, in particular thioacetamide and thiourea, as well asthiolacids (RC═O(SH). The aqueous solution may contain about 0.01 toabout 20, preferably about 0.05 to about 5, and more preferably about0.1 to about 1 gram of sulfide or sulfide precursor (expressed by weightas sulfur) per liter of aqueous medium.

[0012] The colloid stabilizer is a material added to a colloidaldispersion to stabilize the dispersion. Such stabilizers are known inthe general field of colloid chemistry. The colloidal stabilizer may bean inorganic salt of a complex polyacid such as one of Group VA, VB orVIA of the Periodic Table of the Elements, such as phosphorus,molybdenum or tungsten, for example, a polyphosphate or aheteropolyacid. Sodium metal phosphate and sodium tripolyphosphate arepreferred. The colloidal stabilizer may also be a polymeric watersoluble hydrophilic compound, such as a synthetic polymer, for example,polyvinyl alcohol or poly(meth)acrylic acid or polyvinyl pyrrolidone, ora natural polymer, such as vegetable gum, for example, guar gum, orgelatin or xanthan gum. The colloidal stabilizer may be present in theaqueous medium in amounts of about 0.01 to about 30, preferably about0.05 to about 20, and more preferably about 0.05 to about 5 grams perliter of the aqueous medium.

[0013] The complexing agent, which is preferably present in the aqueousmedium, complexes the cadmium ion under the pH conditions in the medium.The complexing agent is usually water soluble and, preferably,nitrogenous, preferably with at least one amino nitrogen atom, forexample 1 to about 4 amino nitrogen atoms, and may be added as free baseor as an amine salt. Thus, the complexing agent may be ammonia, usuallyadded as an ammonium salt, for example a halide such as a chloride, anitrate or a sulphate. It may also be a halide, nitrate or sulfate of aprimary, secondary or tertiary organic amine and, in particular, of analiphatic amine such as an amino alkane, which may also have at leastone hydroxyl substituent. Examples of such organic amines are mono-,di-and triethanolamine and propanolamine. Other suitable amines arealkylene diamines such as ethylene diamine, and amino acids such asethylene diamine tetracetic acid. The concentration of complexing agentin the aqueous medium is usually about 10 to about 200, preferably about40 to about 100 grams per liter of aqueous medium. In particular, theratio of moles of complexing agent to Cd atoms is usually about 1:1 toabout 1000:1, for example, about 50:1 to about 500:1.

[0014] The aqueous medium usually has a pH of about 8 to about 13,preferably about 9 to about 12.5, which may be achieved by addition ofalkali, for example, sodium hydroxide, a basic complexing agent such asammonia, or both.

[0015] The deposition process is usually performed at a temperature ofabout 20 to about 90° C., preferably about 50 to about 80° C., and in atime of about 5 to about 100 minutes, preferably about 10 to about 50minutes. The aqueous medium may be unagitated or may be agitated eitherperiodically or continuously. The process may be performed by mixing allthe components and allowing reaction to occur, but preferably thestabilizer and cadmium ion, optionally with complexing agent, are mixedfirst to give a solution thereof and then to this is added the sulfideor precursor thereof. The preferred method to form a CdS layer for thethin film semiconductor of this invention is to use a chemicaldeposition process where the CdS is deposited by immersing the substratecontaining the first and second transparent layer deposited thereon in abath containing a warm alkaline solution containing the cadmium complex([Cd(NH₃)₄]²⁺ and thiourea. Such processes are disclosed, for example,in N. R. Pavaskar, et al., J. Electrochem. Soc. 124 (1967) p. 743, andin I. Kaur, et al., J. Electrochem. Soc. 127 (1981) p. 943 which areboth incorporated herein by reference. Irrespective of the method used,the CdS layer can be deposited to a thickness of up to about 0.12microns. After deposition of the CdS layer, it is typically heated inair at a temperature of about 300° C. to about 500° C., preferably atabout 350° C. to about 450° C. for about 10 to about 60 minutes, morepreferably about 15 to about 40 minutes, to anneal the CdS layer. Duringthis annealing process it is believed that the CdS layer undergoesdensification and grain growth.

[0016] We discovered that the CdS layer in the thin film semiconductordevice of this invention can be made much thinner than the CdS layers inprior photovoltaic devices containing CdTe and CdS layers. For example,in the thin film semiconductor device of this invention the CdS layer orn-layer can be thin. By thin we mean, preferably, up to about 0.07microns in thickness, more preferably about 0.01 to about 0.065 micronsin thickness, and most preferably about 0.04 to about 0.06 microns inthickness. Such thin CdS layers are desirable because the CdS layer,although a window layer, nevertheless absorbs light energy which webelieve contributes to the lowering of the efficiency of a CdS/CdTephotovoltaic device. Consequently, the thinner CdS layer or n-layer ofthe thin film semiconductor device of this invention when used in aphotovoltaic device results in a more efficient photovoltaic device forconverting light energy into electrical energy.

[0017] The p-layer in the thin film semiconductor device of thisinvention preferably comprises one or more IIB elements and one or moreVIA elements of the Periodic Table of Elements as appearing in “AdvancedInorganic Chemistry” by Cotton and Wilkinson, 4^(th) Edition, in whichthe Group IIB elements include cadmium and the Group VIA elementsinclude selenium and tellurium. The preferred p-layer comprises cadmiumand tellurium which may also contain mercury as disclosed in U.S. Pat.No. 4,548,681 which is incorporated herein by reference. Additionally,the p-layer may contain quantities of dopants such as one or more ofcopper, gold or silver as disclosed in EP Patent 244963 which isincorporated herein by reference. Other p-type layers include, forexample, copper indium diselenide, copper sulfide, copper indiumdisulfide, GaSb, GaAs, Sn1_(±x)Se, InSb, CulnSe_(2−x), and along withCdTe one or more mixtures thereof, or one or more of the p-type layersdisclosed in U.S. Pat. No. 4,753,684 which is incorporated herein byreference. However, the preferred p-layer comprises Cd and Te,preferably CdTe, with or without mercury or the dopants mentioned above.Preferably the CdTe p-layer is deposited by electrodeposition. Asuitable method for electrodeposition of a CdTe layer as well as othersuitable IIB and VIA elements is disclosed in Panicker, et al.,“Cathodic Deposition of CdTe from Aqueous Electrolytes,” J. Electrohem.Soc. 125, No. 4, 1978, pp. 556-572, and in U.S. Pat. No. 4,400,244 whichare both incorporated herein by reference. In this method, deposition ofCdTe takes place from an aqueous solution of CdSO₄ to which TeO₂ hasbeen added and the electrodeposition is carried out onto the substratehaving the first and second transparent layers, and the CdS layerdeposited thereon. Preferably, in the solution the concentration of Cd²⁺ions is about 0.2 to about 1.5 molar, and the concentration of HTeO₂+ions is about 10⁻⁵ molar to about 10⁻³ molar, and the pH of the solutionis suitably about 1 to about 3, and is conveniently adjusted by an acidsuch as sulfuric or hydrochloric acid. In such an electrochemicalmethod, HTeO₂+ at the cathode reacts with Cd²+ ions to form cadmiumtelluride which is deposited on the glass substrate cathode. In thesemiconductor device of this invention, the p-layer having IIB and VIAelements, for example, a p-layer of CdTe, copper indium diselenide,GaSb, GaAs, Sn_(1±x)Se, InSb, CulnSe_(2−x) or one or more mixturesthereof, and particularly a CdTe p-layer, is preferably deposited byelectrodeposition directly onto and in direct contact with the CdSwindow layer. Methods for the electrodeposition of suitable p-layers orprecursors therof useful in this invention are also disclosed in EPPatent 0538041 which is incorporated herein by reference.

[0018] A CdTe layer deposited electrolytically as described above hasn-type conductivity and, therefore, cannot form a rectifyingheterojunction with a CdS n-type layer capable of generating electricalenergy upon exposure to light renergy. To produce a rectifying junction,the n-type CdTe layer is heat treated in air at, for example, atemperature of about 250 to about 500° C. for a time sufficient, forexample, about 5 to about 10 minutes, to convert the n-type CdTe layerto a relatively low resistivity p-type layer. Such a heating process isdisclosed in U.S. Pat. No. 4,388,483 which is incorporated herein byreference.

[0019] The p-type layer in the thin film semiconductor device of thisinvention is typically up to about 5 microns in thickness, preferablyabout 0.5 microns to about 3.0 microns in thickness, and more preferablyabout 1.5 to about 2.5 microns in thickness. During the heat treatmentof the CdTe layer as described above, the CdTe layer preferablyrecrystallizes and undergoes grain growth.

[0020] The semiconductor devices of this invention, if used as aphotovoltaic device, generally have a back contact. This back contact ispreferably deposited on and in direct contact with the p-layer. The backcontact is suitably made from one or more highly conductive materials.The conductive back contact may be, for example, one or more ofelemental nickel, chromium, copper, tin, aluminum, gold, silver,technecium or alloys or mixtures of any of the above such as, forexample, an alloy of tin and zinc. It can be layers of one or moremetals such as the metals just mentioned, for example a layer of nickeland a layer of chromium. It can be made from blends of graphite andpolymeric materials, carbon pastes and, it can also be a transparentconductive oxide such as, for example, the conductive oxides describedhereinabove useful for the first transparent layer. The back conductivecontact can be a layer of carbon deposited on the p-layer followed byone or more layers of metal, such as the metals described above. Theback conductive contact, if made of or comprising one or more metals, issuitably applied by a technique such as sputtering or metal evaporation.If it is made from a graphite and polymer blend, or from a carbon paste,the blend or paste is applied to the semiconductor device by anysuitable method for spreading the blend or paste, such as screenprinting, spraying or by a “doctor” blade. After the application of thegraphite blend or carbon paste, the device is heated to convert theblend or paste into the conductive back contact layer. Suitablecarbon-containing pastes or inks can be obtrained from suppliers such asDuPont Microcircut Materials, Mettech Polymers Group, Acheson ColloidsCompany, and Coates Circuit Products. Suitable back contacts aredisclosed in U.S. Pat. No. 4,735,662 which is incorporated herein byreference. A carbon layer, if used, is suitably about 1 to about 10microns in thickness. A metal layer of the back contact, if used for oras part of the back contact, is suitably about 0.1 to about 1 microns inthickness. Prior to adding the back conductive contact, the p-layer maybe treated as set forth in U.S. Pat. Nos. 4,456,630 and 5,472,910 whichare incorporated herein by reference. These references teach methods todope the p-layer and methods to improve the ohmic contact between thep-type layer and the conductive back contact.

[0021] When using the semiconductor device of this invention as aphotovoltaic device, it is useful to connect a plurality of the devicesin series in order to achieve a desired voltage. Any suitable method canbe used to accomplish such a connection, for example, electrical wiringor other conductive means can be used to connect a plurality of devicesin series. Each end of the series connected cells can be attached to asuitable conductor such as a wire or bus bar, to direct thephotovoltaically generated current to convenient locations forconnection to a device or other system using the generated electric. Aconvenient means for achieving such series connections is to laserscribe the device to divide the device into a series of cells connectedby interconnects. Methods for interconnecting cells in a seriesconfiguration are disclosed in U.S. Pat. Nos. 4,243,432 and 4,383,022which are incorporated herein by reference. Preferably, a laser is usedto scribe the deposited layers of the semiconductor device to divide thedevice into a plurality of series connected cells. A laser scribingprocess is disclosed in the article by D. Cunningham et al., “Large AreaApollo Module Performance And Reliability,” 28^(th) IEEE PhotovoltaicSpecialists Conference, Anchorage, Ak. September 2000, which isincorporated herein by reference.

[0022] One embodiment of the invention will now be described withreference to FIG. 1.

[0023]FIG. 1 shows the layered structure of one embodiment of a thinfilm, layered semiconductor 1 of this invention. The layers are notdrawn to scale in FIG. 1 with respect to the relative thickness of eachlayer.

[0024] In FIG. 1, 2 is a glass substrate, preferably a flat glasssubstrate made from a high quality float glass. Layer 3 is a firsttransparent conductive layer comprising tin oxide doped with fluorineatoms to make it conductive. Layer 3 has a thickness of about 0.5microns and is preferably deposited on the glass substrate by CVD.Alternatively, glass substrate 2 can be purchased from glass suppliershaving the layer 3 deposited thereon. Layer 4 is a second transparentlayer having a resistivity more than the resistivity of conductive layer3. Layer 4, for example, has a resistivity of about 1 to about 100ohm·cm. Layer 4 is a mixture of tin and zinc oxides in a molar ratio of99:1 formed by reactive DC sputtering and is about 0.03 microns inthickness.

[0025] Layer 5 is a CdS n-type or window layer formed by chemicaldeposition from a bath of a warm alkaline solution of cadmium complex([Cd(NH₃)₄]²⁺ and thiourea. The deposited CdS is heated in air at about400° C. for about 30 minutes after deposition. Layer 6 is a p-type layerof CdTe formed by electrodeposition from an acidic bath of Cd²⁺ andTeO₂. After electrodeposition, the deposited CdTe layer is heated orannealed in air at a temperature of about 450° C. for about 20 minutesto convert it to the desired p-layer. Upon such heating, thephotovoltaically active heterojunction depicted by line 7 is formedbetween the n-type, CdS layer and the p-type CdTe layer. Layer 8 is aconductive back contact made of carbon formed by applying and heating acarbon-containing ink. Layer 9 is a layer of metal, such as tin, addedby sputtering and is about 0.25 to about 0.75 microns in thickness.

[0026] Although not wishing to be bound by any theory of operation, webelieve that the thin, second transparent layer in the semiconductordevice of this invention, and in the photovoltaic device made therefrom,reduces electrical shunting of what would otherwise occur between theCdTe p-layer and the first transparent layer. In prior devices, defectsin the CdS n-layer may cause such shunting. In order to reduce theshunting, the thickness of the CdS layer was typically increased.However, upon increasing the thickness of the window CdS layer, there isa concomitant decrease in the efficiency of the photovoltaic device dueto the absorption of light by the thick CdS layer. In the semiconductordevice of this invention, the thin second transparent layer is believedto alleviate such shunting because of its lower conductivity or higherresistivity. Thus, even though the thinner CdS layer may still havedefects, the shunting is reduced or eliminated by the presence of thethin, higher resistivity second transparent layer.

[0027] In the semiconductor device of this invention, the secondtransparent layer, although having a conductivity less than theconductivity of the first transparent layer, has a conductivitypreferably sufficiently high to permit the electrodepositon of a CdTelayer as described hereinabove. For example, if the resistivity of thesecond transparent layer is too large, for example, more than about 15ohm·cm, or more than about 10 ohm·cm, a CdTe layer deposited by theelectrochemical methods as described herein, for example, may benon-stoichiometric. By non-stoichiometric we mean it will not have thepreferred Cd:Te atomic ratio. A non-stoichiometric CdTe layer may leadto a less than optimal CdTe absorber layer for conversion of solarenergy or other light energy to electrical energy. Thus, the secondlayer is preferably of a thickness and resistivity to reduce oreliminate shunting associated with thin CdS layer yet of a sufficientconductivity to provide for the prefered stoichiometry of the CdTe layerformed by an electrolytic deposition process. By prefered stoichiometrywe mean, preferably, a stoichiometry where the atomic ratio of Cd to Teis within the range Cd_(1+x)Te_(1+y), where x and y are no more thanabout ±0.01. Stated differently the CdTe stoichiometry should be within1% of the 1:1 stoichiometry. Preferably the atomic ratio of Cd to Te issubstantially 1:1, and most preferably the atomic ratio of Cd to Te is1:1 in the CdTe layer of the semiconductor device of this invention.

[0028] We have also found the thin second transparent layer of thesemiconductor device of this invention provides for an improvedphotovoltaic device, e.g., a device with higher efficiency, compared toa photovoltaic device having a thicker second transparent layer.Photovoltaic devices of this invention comprising the semiconductordevice of this invention have an improved Isc without loss of opencircuit voltage (Voc) and fill factor (FF) compared to prior artCdS/CdTe photovoltaic devices.

[0029] This invention is also a method for making the thin filmsemiconductor devices of this invention. The method comprises depositingon a substrate, suitably a glass substrate, a first transparent layer asdescribed hereinabove comprising a transparent conductive material asdescribed hereinabove; depositing a second transparent layer asdescribed hereinabove having a conductivity less than the conductivityof the first transparent layer; depositing an n-type, preferably thin,layer as described hereinabove; depositing a p-type layer as describedhereinabove or, as also described hereinabove, a layer which can beconverted into a p-type layer after deposition; and depositing a second,generally opaque, conductive layer or layers as described above whichcan serve as an electrical contact. Photovoltaic devices comprising thethin film semiconductor devices of this invention are highly efficientin converting light energy into electrical energy. For example,photovoltaic devices of this invention comprising the thin layersemiconductor devices of this invention have efficiencies of at leastabout 8.5 percent, of at least about 9.0 percent, of at least about 9.5,of at least about 10, or of at least about 10.5 percent. Efficiencies ofat least about 11 or about 11.5 percent can be achieved. Photovoltaicdevices comprising the semiconductor devices of this invention haveefficiencies of about 11.5 percent to about 8.5 percent, for exampleabout 11 percent to about 9 percent. The efficiency of a photovoltaicdevice of this invention made using the semiconductor device of thisinvention can be conveniently and preferably is measured according toASTM E-948-95.

[0030] The following examples are being provided to illustrate certainembodiments of the invention, however, they are not intended to limit inanyway the scope thereof.

[0031] Provisional patent application 60/289,481 filed on May 8, 2001,is incorporated by reference in its entirety. All references to thePeriodic Table of Elements hereinabove are to the Periodic Table ofElements as appearing in “Advanced Inorganic Chemistry,” Cotton andWilkinson, 4^(th) Ed.

EXAMPLES

[0032] Thin film photovoltaic devices were made as follows and testedfor efficiency in converting light energy to electrical energy. Thephotovoltaic devices tested had a 3 mm thick float glass sheet as asubstrate material. The substrate was coated with a layer of transparentconducting tin oxide about 0.5 to 0.6 microns thick. The conductive tinoxide was applied using chemical vapor deposition and had a resistivityof about 1×10⁻⁴ ohm·cm. A second transparent layer of tin and zinc oxidein a molar ratio of 99 to 1 was deposited on the first transparent layerusing reactive DC magnetic sputtering in an oxygen atmosphere at apressure of about 1 millitorr. The thickness of the second transparentlayer was varied as shown in the Table 1. After deposition, the secondtransparent layer was heated in air at a temperature of 500° C. for 20minutes. The second transparent layers had resistivities that were aboutthe same. A 0.05 microns film of cadmium sulfide was deposited on thesecond transparent layer by chemical deposition by the reaction ofcadmium ion with thiourea in an aqueous ammonia solution at atemperature of about 70° C. The cadmium sulfide layer was heated in airat 400° C. for 30 minutes after deposition. A layer of cadmium telluridewas deposited on the cadmium sulfide layer by electrodeposition from abath of cadmium sulfate and tellurium dioxide. Chloride ion was alsopresent and was incorporated in the cadmium telluride layer during theelectrodeposition process. The electrodeposited cadmium telluride layerwas about 1.8 microns in thickness after deposition. After deposition,the substrate containing the deposited layers was heated in air at 450°C. for 15 minutes. After heat treatment, the cadmium telluride layer wasdoped with copper at 200° C. A carbon layer was applied to thecopper-doped cadmium telluride by screen printing a layer of carbon ink,and the ink was heated at 100-200° C. in air to form a carbon layerabout 10 microns in thickness. The carbon layer was then coated withaluminum by sputtering a 0.3 microns thick layer of aluminum.

[0033] Table 1 reports the efficiency of the photovoltaic devices soformed. TABLE 1 Thickness of Second Transparent Layer^(a) % Efficiency0.10 2.4 0.075 5.1 0.051 7.5 0.025 8.9

[0034] The results in Table 1 show an increase in efficiency of the thinfilm photovoltaic devic as the second transparent layer is made thinner.

[0035] Only certain embodiments of the invention have been set forth andalternative embodiments and various modifications will be apparent fromthe above description to those of skill in the art. These and otheralternatives are considered equivalents and within the spirit and scopeof the invention.

That which is claimed is:
 1. A layered, thin film semiconductor devicecomprising a first transparent layer comprising a conductive material, athin, second transparent layer having a resistivity greater than thefirst transparent layer, an n-type layer, and a p-type layer comprisingone or more IIB and VIA elements.
 2. The semiconductor device of claim 1wherein the second transparent layer is up to about 0.075 microns inthickness.
 3. The semiconductor device of claim 2 wherein the secondtransparent layer comprises tin oxide.
 4. The semiconductor device ofclaim 3 wherein the second transparent layer further comprises zincoxide.
 5. The semiconductor device of claim 1 wherein the secondtransparent layer is textured.
 6. The semiconductor device of claim 1wherein the second transparent layer is deposited by reactive DCsputtering.
 7. A photovoltaic device comprising the thin filmsemiconductor of claim
 1. 8. The thin film semiconductor device of claim2 wherein the n-layer comprises cadmium sulfide and the p-layercomprises cadmium telluride.
 9. A photovoltaic device comprising thethin film semiconductor of claim
 8. 10. The thin film semiconductor ofclaim 8 wherein the n-layer is up to about 0.07 microns in thickness.11. A photovoltaic device having an efficiency of at least about 8.5percent.
 12. A method for making a thin film semiconductor devicesuitable for use in a photovoltaic device, the method comprising (a)forming on a substrate a first transparent layer comprising a conductivematerial, (b) forming a thin second transparent layer having aresistivity more than the resistivity of the first transparent layer,(c) forming an n-type layer or precursor layer thereof, and (d) forminga p-type layer.
 13. The method of claim 12 wherein the secondtransparent layer is deposited on and is in direct contact with thefirst transparent layer.
 14. The method of claim 13 wherein the n-typelayer is deposited on and is in direct contact with the secondtransparent layer.
 15. The method of claim 14 wherein the p-type layeris formed on and is in direct contact with the n-type layer.
 16. Themethod of claim 15 wherein the second transparent layer is up to about0.075 microns in thickness.
 17. The method of claim 16 where the secondconductive layer comprises tin oxide.
 18. The method of claim 17 whereinthe second conductive layer is deposited using reactive DC sputtering.19. The method of claim 16 wherein the n-type layer comprises cadmiumsulfide.
 20. The method of claim 19 wherein the cadmium sulfide n-typelayer is up to about 0.07 microns in thickness.
 21. The method of claim12 wherein the second transparent layer has a resistivity that providesfor the deposition of a CdTe layer in a stoichiometry of about 1:1 byelectrochemical deposition.
 22. The semiconductor device of claim 1wherein the resistivity of the second transparent layer is no more thanabout 15 ohm·cm.
 23. The semiconductor device of claim 1 wherein theresistivity of the second transparent layer is no more than about 10ohm·cm.